This invention relates to a combined steam turbine and gas turbine power plant for generating electric power. More specifically, if concerns a combined cycle power plant wherein a single carbonaceous fuel such as sulfur-bearing coal serves as the source of energy for driving thermodynamically coupled steam and gas turbines.
The basic concept of utilizing Rankine-cycle steam turbines and Brayton-cycle gas turbines in thermodynamically coupled combined cycle arrangements to achieve overall plant efficiencies higher than would be obtainable for either power cycle alone (operating over the same range of working fluid temperatures and pressures) is well known. Thermal energy which would be rejected from a simple cycle Brayton gas turbine plant and lost is utilized in combined cycle configurations to heat feedwater or steam for the Rankine-cycle steam turbine thereby achieving greater combined power generation for a given calorific input. The higher plant efficiencies achievable with combined cycles reduce fuel requirements and costs, as well as the quantity of undesirable plant effluents associated with the discharge of combustion products and reject heat to the environment. Since most fuels used for combustion power plants come from depletable fossil reserves, higher plant efficiencies are also desirable to conserve such reserves.
One of the present limitations on the use of integrated, high-efficiency combined cycle plants arises from the present purity requirements of gases used in gas turbines to avoid hot corrosion, fouling and rapid deterioration of gas turbine parts, particularly gas turbine blade surfaces. Current combined cycle plants are usually limited to burning natural gas or refined or specially treated petroleum fuels, which, unfortunately, are rapidly becoming less competitive in availability and price. Increasingly, in areas such as the United States, the use of coal for electric power generation rather than natural gas or petroleum is sought since indigenous coal reserves are much larger than those of oil or natural gas. While conventional Rankine-cycle steam power plants are more tolerant than gas turbines of combustion products due to fuel impurities, their efficiency is limited and in recent years has been reduced by the energy requirements of pollution control equipment which has been necessary, particularly when dirtier fuels such as high-sulfur coal is burned.
Various suggestions have been made to obtain the benefits of use of a single fuel such as coal as the source of energy in a combined steam and gas turbine cycle plant. One proposal, exemplified by U.S. Pat. Nos. 3,234,735 issued to Pirsh et al. and 3,990,229 issued to Staege, is to produce coal gas in the burning or partial oxidation of coal, pass the gas through cleaning and compression units, then into a gas turbine combustor for burning and subsequent expansion through a gas turbine. A significant disadvantage of this technique is that it requires rather complex gasifier equipment and cleaning apparatus, thus adding to overall plant cost. In addition, substantial losses in heating value of the gasified fuel are suffered in the coal to gas conversion. Use of gasifiers may also entail operating risks.
The large fuel conversion losses and other specific disadvantages of combined cycles which gasify and clean the primary fuel before combustion can be avoided in combined cycle systems which employ a furnace pressurized by the gas turbine compressor and a pressure-letdown gas turbine to recover energy from the expanding combustion gases. These systems are described and analyzed, for example, in the U.S. Government report NASA-CR 134949, "Energy Conversion Alternatives Study" (see Volume 1, FIG. 2, Page 14, for instance). A major obstacle to the widespread application of such pressurized furnace combined cycle systems is the difficult and costly system required to clean the hot combustion gases to a sufficient level that corrosion and fouling of the gas turbine blading and other system components is not excessive.
Another method of avoiding corrosion of turbine blading by coal combustion gases in a combined cycle is disclosed in U.S. Pat. No. 3,127,744 issued to Nettel wherein recirculating pebbles heated in a secondary furnace are arranged to cascade downward in counterflow to compressed air transferring heat thereto, the heated air then driving a gas turbine. Drawbacks associated with this system include the requirement of secondary burner, a rather complex pebble pump and distribution system, and the probable need of frequent scrubbing or cleaning of the pebbles to avoid contamination of airflow to the gas turbine. Moreover, although this system ostensibly permits operation with a variety of fuels, it does not address the problem of cleaning sulfur and other contaminants from the furnace exhaust gases.
Yet another proposed combined cycle arrangement is described in the Dec. 15, 1976 issue of Electrical World, pp. 39-41 in an article "Fluid-bed Technology Advances". In the cycle there suggested for use in a Curtiss-Wright pilot plant, coal is burned in a pressurized fluidized bed combustor with one-third of the gas turbine compressor airflow used as combustion air and the remaining two-thirds passing through a heat exchanger within the bed, the flows subsequently being recombined after cleaning of the combustion gases and then expanded through a gas turbine. One significant disadvantage of this configuration in common with most of the other arrangements noted previously is the cost of cleaning equipment for removing impurities from the hot stream of combustion gases passing in contact with the bed. Moreover, high levels of efficiency of this power plant are difficult to maintain as operating load is decreased because the fluidized bed temperature and hence the energy produced by the gas turbine tends to drop rapidly as the gas turbine compressor flow is reduced in correspondence with reduced fuel flow.
Accordingly, it is a general object of the invention to provide an economic combined cycle steam turbine and gas turbine power plant with high efficiency.
Another object of the invention is to provide an improved combined cycle steam turbine and gas turbine power plant which burns sulfur-bearing carbonaceous fuel such as coal in an environmentally clean manner without costly cleanup.
A further object of the invention is to provide an improved coal-fired combined cycle steam turbine and gas turbine power plant wherein unreliability due to gas turbine corrosion and fouling is avoided in a relatively simple arrangement of apparatus.